When I first heard a fairly superficial list of similarities between Forrest Gump and The Curious Case Of Benjamin Button, it didn't strike me as particularly compelling. I don't really care that they both worked on boats or they both went to war: such are the realities of Guy Recounts His Long Life movies.
It's the tone and the self-important pronouncements about life -- many of which, in Button's case, make absolutely no sense given the actual details of the movie's conceit -- that make them feel like the same movie. (And they are, as many have already pointed out, from the same screenwriter.) The above video from FunnyOrDie.com makes a pretty good jab at this issue, although without hearing Brad Pitt's syrupy "Well ah guess ah wuz just special"-style narration, you really can't grasp how similar they feel from a theater seat.